Many industrial processes use synthesis gas, which is otherwise known as syngas, for power generation, chemical synthesis, or other applications. Syngas primarily consists of hydrogen gas and carbon monoxide. Syngas can be produced from most kinds of hydrocarbon, including, but not limited to coal, oil and natural gas, by the application of high temperature and steam. Syngas can be burned, separated into carbon monoxide and hydrogen, or converted to a liquid fuel by processes like the Fischer-Tropsch process. In addition, syngas can be converted into methane, methanol, ammonia and other chemicals. The production of syngas can create substantial polluting emissions, such as, but not limited to, H2S and CO2. Combustion of syngas can also produce other polluting emissions, including NOx.
Conventional gas cleaning technologies are often used to remove these contaminants; however, disposal of the byproducts of the cleaning process can be expensive. For instance, conventional gas cleaning technologies for coal derived gases typically include a series of water scrubbers and low-temperature solvent absorption gas cleaning steps to reduce the concentration of the contaminants to an acceptable level for chemical synthesis, such as methanol production, for integrated gasification combined cycle (IGCC) power generation applications, or for other applications.
Typically, a conventional syngas cleaning process begins with cooling raw syngas to a relatively low temperature. The syngas is then passed through a water venturi scrubber to remove particulates from the syngas. The syngas is further cooled to form a condensate stream and a nearly dry syngas that is scrubbed with water to remove ammonia and halides from the syngas. This operation generates highly contaminated water requiring considerable treatment. The temperature of the syngas is then reduced to about 100 degrees Fahrenheit to reduce the concentration of mercury to a desired amount by passing the syngas through a packed bed of sulfur-impregnated activated carbon, which is an expensive sorbent.
The dry syngas is often chilled or refrigerated and scrubbed with an absorbent to remove sulfur contained in the gas. Some absorbents may require hydrolysis conversion of COS to H2S to achieve high levels of sulfur removal. One of the most effective absorbents for removing sulfur is methanol at temperatures as low as −100 degrees Fahrenheit. Methanol is used in the Rectisol desulfurization process. The Rectisol desulfurization process does not require hydrolysis of COS to H2S and can reduce sulfur concentrations to relatively low levels in syngas. However, the Rectisol process requires a substantial investment in equipment and incurs high power costs. In addition, the Rectisol process effectively and undesirably for IGCC removes CO2 from the syngas.
The syngas is then reheated to about 700 degrees Fahrenheit and passed through a fixed bed of sulfur sorbent, such as a ZnO guard bed, to further reduce the sulfur concentration in the syngas. Reheating the syngas consumes energy and is inefficient.
Many conventional syngas cleaning processes function well, and some can meet very stringent levels of contaminant removal need for chemical synthesis. However, most are expensive to construct and operate. In addition, most systems require complex water treatment systems to remove the contaminants from the water generated during the cleaning process. Moreover, many of the conventional systems result in “drying” of the gas and partial CO2 removal, which results in an energy loss that is not acceptable for some applications.
In addition, conventional syngas often contains products that can harm power generation in IGCC applications such as metal cabonyls and particles from corrosion of the syngas pipes. In particular, these products can be deposited in the turbines and can cause erosion damage in the turbines.
Thus, a need exists for a more efficient system and method for cleaning gas.